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Sweet spots in the sea: Moun­tains of sugar un­der seagrass mead­ows

May 2, 2022
Seagrass mead­ows are un­der­wa­ter oases. Now re­search­ers have dis­covered vast amounts of sug­ars un­der­neath seagrass mead­ows. This sheds new light on how plants store car­bon in the ocean.

Scientists from the Max Planck Institute for Marine Microbiology now report that seagrasses release large amounts of sugar, largely in the form of sucrose, into their soils – worldwide more than 1 million tons of sucrose, enough for 32 billion cans of coke. Such high concentrations of sugar are surprising. Normally, microorganisms quickly consume any free sugars in their environment. The scientists found that seagrasses excrete phenolic compounds, and these deter most microorganisms from degrading the sucrose. This ensures that the sucrose remains buried underneath the meadows and cannot be converted into CO2 and returned to the ocean and atmosphere. They now describe their discovery in the journal Nature Eco­logy & Evol­u­tion.

Üppige Seegraswiesen
Üppige Seegraswiesen von Posidonia oceanica im Mittelmeer. Die Forschenden des Max-Planck-Instituts für Marine Mikrobiologie gehen davon aus, dass ihre Erkenntnisse für viele Lebensräume von Meerespflanzen relevant sind, beispielweise andere Seegrasarten, Mangroven und Salzwiesen. (© HYDRA Marine Sciences GmbH)

Seagrasses form lush green mead­ows in many coastal areas around the world. These mar­ine plants are one of the most ef­fi­cient global sinks of car­bon di­ox­ide on Earth: One square kilo­meter of seagrass stores al­most twice as much car­bon as forests on land, and 35 times as fast. Now sci­ent­ists from the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy in Bre­men, Ger­many, have dis­covered that seagrasses re­lease massive amounts of sugar into their soils, the so-called rhizo­sphere. Sugar con­cen­tra­tions un­der­neath the seagrass were at least 80 times higher than pre­vi­ously meas­ured in mar­ine en­vir­on­ments. “To put this into per­spect­ive: We es­tim­ate that world­wide there are between 0.6 and 1.3 mil­lion tons of sugar, mainly in the form of sucrose, in the seagrass rhizo­sphere”, ex­plains Manuel Liebeke, head of the Re­search Group Meta­bolic In­ter­ac­tions at the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy. “That is roughly com­par­able to the amount of sugar in 32 bil­lion cans of coke!”

 

Poly­phen­ols keep mi­crobes from eat­ing the sugar

HYDRA
Beautiful to look at, hard to sample: Measuring metabolites like sucrose and polyphenols in seawater is difficult. The scientists from the Max Planck Institute for Marine Microbiology in Bremen had to develop a special method to deal with the large amounts of salt in seawater that make measurements of metabolites so difficult. (© HYDRA Marine Sciences GmbH)

Mi­crobes love sugar: It is easy to di­gest and full of en­ergy. So why is­n't the sucrose con­sumed by the large com­munity of mi­croor­gan­isms in the seagrass rhizo­sphere? “We spent a long time try­ing to fig­ure this out”, says first au­thor Mag­gie So­gin, who led the re­search off the Italian is­land of Elba and at the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy. “What we real­ized is that seagrass, like many other plants, re­lease phen­olic com­pounds to their sed­i­ments. Red wine, cof­fee and fruits are full of phen­olics, and many people take them as health sup­ple­ments. What is less well known is that phen­olics are an­ti­mi­cro­bi­als and in­hibit the meta­bol­ism of most mi­croor­gan­isms. “In our ex­per­i­ments we ad­ded phen­olics isol­ated from seagrass to the mi­croor­gan­isms in the seagrass rhizo­sphere – and in­deed, much less sucrose was con­sumed com­pared to when no phen­olics were present.”

Some spe­cial­ists thrive on sug­ars in the seagrass rhizo­sphere

Why do seagrasses pro­duce such large amounts of sug­ars, to then only dump them into their rhizo­sphere? Nicole Du­bilier, Dir­ector at the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy ex­plains: “Seagrasses pro­duce sugar dur­ing pho­to­syn­thesis. Un­der av­er­age light con­di­tions, these plants use most of the sug­ars they pro­duce for their own meta­bol­ism and growth. But un­der high light con­di­tions, for ex­ample at mid­day or dur­ing the sum­mer, the plants pro­duce more sugar than they can use or store. Then they re­lease the ex­cess sucrose into their rhizo­sphere. Think of it as an over­flow valve”.

In­triguingly, a small set of mi­cro­bial spe­cial­ists are able to thrive on the sucrose des­pite the chal­len­ging con­di­tions. So­gin spec­u­lates that these sucrose spe­cial­ists are not only able to di­gest sucrose and de­grade phen­olics, but might provide be­ne­fits for the seagrass by pro­du­cing nu­tri­ents it needs to grow, such as ni­tro­gen. “Such be­ne­fi­cial re­la­tion­ships between plants and rhizo­sphere mi­croor­gan­isms are well known in land plants, but we are only just be­gin­ning to un­der­stand the in­tim­ate and in­tric­ate in­ter­ac­tions of seagrasses with mi­croor­gan­isms in the mar­ine rhizo­sphere”, she adds.

En­dangered and crit­ical hab­it­ats

Liebeke and Dubilier
Manuel Liebeke and Nicole Dubilier in front of the Imaging mass spectrometer at the Max Planck Institute in Bremen, an instrument that was essential for the current research. (© Achim Multhaupt)

Seagrass mead­ows are among the most threatened hab­it­ats on our planet. “Look­ing at how much blue car­bon – that is car­bon cap­tured by the world's ocean and coastal eco­sys­tems – is lost when seagrass com­munit­ies are decim­ated, our re­search clearly shows: It is not only the seagrass it­self, but also the large amounts of sucrose un­der­neath live seagrasses that would res­ult in a loss of stored car­bon. Our cal­cu­la­tions show that if the sucrose in the seagrass rhizo­sphere was de­graded by mi­crobes, at least 1,54 mil­lion tons of car­bon di­ox­ide would be re­leased into the at­mo­sphere world­wide”, says Liebeke. "That's roughly equi­val­ent to the amount of car­bon di­ox­ide emit­ted by 330,000 cars in a year." Seagrasses are rap­idly de­clin­ing in all oceans, and an­nual losses are es­tim­ated to be as high as 7% at some sites,­ com­par­able to the loss of coral reefs and trop­ical rain­forests. Up to a third of the world’s seagrass might have been already lost. “We do not know as much about seagrass as we do about land-based hab­it­ats”, So­gin em­phas­izes. “Our study con­trib­utes to our un­der­stand­ing of one of the most crit­ical coastal hab­it­ats on our planet, and high­lights how im­port­ant it is to pre­serve these blue car­bon eco­sys­tems."

Ori­ginal pub­lic­a­tion

 

 

E. Mag­gie So­gin, Dolma Michel­lod, Har­ald Gruber-Vodicka, Patric Bourceau, Be­ne­dikt Geier, Di­mitri V. Meier, Mi­chael Seidel, So­eren Ah­merkamp, Sina Schorn, Grace D‘An­gelo, Gab­ri­ele Proc­ac­cini, Nicole Du­bilier, Manuel Liebeke: Sug­ars dom­in­ate the seagrass rhizo­sphere. Nature Eco­logy & Evol­u­tion (2022)

DOI: 10.1038/s41559-022-01740-z

Par­ti­cip­at­ing in­sti­tu­tions

  • Max Planck Institute for Marine Microbiology, Bremen, Germany
  • University of California at Merced, CA, USA
  • University of Vienna, Vienna, Austria
  • University of Oldenburg, Oldenburg, Germany
  • Stazione Zoologica Anton Dohrn Napoli, Naples, Italy

Please dir­ect your quer­ies to:

Group leader

Research Group Metabolic Interactions

Dr. Manuel Liebeke

MPI for Marine Microbiology
Celsiusstr. 1
D-28359 Bremen
Germany

Room: 

3244

Phone: 

+49 421 2028-8220

Dr. Manuel Liebeke

Director

Department of Symbiosis

Prof. Dr. Nicole Dubilier

MPI for Marine Microbiology
Celsiusstr. 1
D-28359 Bremen
Germany

Room: 

3241

Phone: 

+49 421 2028-9320

Prof. Dr. Nicole Dubilier

Dr. E. Maggie Sogin

As­sist­ant Pro­fessor

Uni­versity of Cali­for­nia, Merced, USA

Phone: +1-508-566-5933

Email: eso­gin@ucmerced.edu

Head of Press & Communications

Dr. Fanni Aspetsberger

MPI for Marine Microbiology
Celsiusstr. 1
D-28359 Bremen
Germany

Room: 

1345

Phone: 

+49 421 2028-9470

Dr. Fanni Aspetsberger
 
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